1. Field of the Invention
The present invention relates to a light emitting diode (LED) driving technology, more particularly, to an LED driving apparatus with protection function.
2. Description of the Related Art
FIG. 1 is a diagram of a conventional LED driving apparatus 10. Referring to FIG. 1, the LED driving apparatus 10 is suitable for driving the LED string 101 including a plurality of LEDs L connected in series. The LED driving apparatus 10 includes a control chip 103, an external circuit 105, a power switch Q and a resistor Rcs. The control chip 103 is used for generating a driving signal VPW in response to the voltage Vcs on the node N2, so as to switch the power switch Q, and thus making the LED string 101 to be operated under a constant current for producing light.
The external circuit 105 is composed of a Zener diode ZD, a resistor R, a capacitor C and a comparator CMP. During an activation phase of the LED driving apparatus 10, the Zener diode ZD, the resistor R and the capacitor C are used for detecting the voltage VD on the node N1, so as to generate a detection voltage VSLP. When the DC voltage VBUS is greater than the voltage value Vz of the Zener diode ZD, the value of the detection voltage VSLP is equal to Iz*R (i.e. VSLP=Iz*R), where Iz is a current flowing through the Zener diode ZD, and R is the resistance of the resistor R.
Meanwhile, once the comparator CMP compares that the detection voltage VSLP is greater than the predetermined VSET, namely, a part of or all of LEDs L in the LED string 101 are short circuited, the comparator CMP would output a fault signal FS to the control chip 103, so as to make the control chip 103 stop generating the driving signal VPW, and thus protecting the control chip 103 and the power switch Q from damage due to a high voltage (i.e. the DC voltage VBUS).
However, the structure of the conventional LED driving apparatus 10 has some problems as below.
1. In some situations, for example, all of LEDs L in the LED string 101 are short circuited, or the power switch Q is turned off during the shut-down phase of the LED driving apparatus 10, the voltage VD on the node N1 would be higher (a couple of hundred volts). In such case, the Zener diode ZD must be used to block the high voltage (i.e. the DC voltage VBUS) to protect the comparator CMP and/or the control chip 103 both with low voltage process.
2. The voltage value Vz of the Zener diode ZD must be changed with the DC voltage VBUS, for example, when the DC voltage VBUS is 200 volts, the voltage value Vz of the Zener diode ZD must be 200 volts, and so on. Therefore, the Zener diode ZD cannot be integrated with the control chip 103 in the structure of the conventional LED driving apparatus 10.